Apple's Shipping Track Record

i see a lot of people saying  is giving themselves buffer room on the  watch shipping and hopefully they ship sooner than the date given. What is apples track record on doing that like I've never run into a issue when i bought iPhones i always got them on day released when I preordered.

Has  in the past had times when they gave them weeks of buffer room and then they got their apple device sooner than expected?